File.Move doesn't overwrite targets

File.Move will throw an exception whenever the target file exists.
Assuming that the task should overwrite the existing target files
for smooth operation, this change checks whether the target file
exists and, if so, deletes it prior to calling File.Move

Author: grendello

src/Xamarin.Android.Tools.BootstrapTasks/Xamarin.Android.Tools.BootstrapTasks/UnzipDirectoryChildren.cs

@@ -112,8 +112,11 @@ async TTask ExtractFile (string tempDir, string sourceFile, string relativeDestD
 					Log.LogMessage (MessageImportance.Low, $"mv '{fse}' '{dest}'");
 					if (Directory.Exists (fse))
 						Process.Start ("/bin/mv", $@"""{fse}"" ""{dest}""").WaitForExit ();
-					else
+					else {
+						if (File.Exists (dest))
+							File.Delete (dest);
 						File.Move (fse, dest);
+					}
 				}
 			}
 		}